This invention generally relates to leak detection and more particularly relates to a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom.
However, before discussing the current state of the art, it is instructive first to briefly describe the structure and operation of a typical nuclear reactor which may contain leaking fuel rods. In this regard, a typical nuclear reactor includes a pressure vessel containing a plurality of fuel assemblies therein defining a nuclear reactor core capable of producing heat by fission of fissile nuclear material. More specifically, each fuel assembly includes a plurality of fuel rods having the fissile nuclear material sealingly contained therein in the form of a plurality of coaxially stacked fuel pellets. As the fissile material forming the fuel pellets fissions, radioactive fission products (e.g., Xenon - 135 and Kr-85) are produced within the fuel rod. Such radioactive fission products are normally sealingly contained within the fuel rod because the fuel pellets producing the fission products are themselves sealingly contained within the fuel rod. Liquid moderator coolant (i.e., demineralized water) is caused to flow through the fuel assemblies and over the fuel rods for assisting in the fission process and for removing the heat produced by the fission process. During operation of the nuclear reactor, the heat due to fission of the fissile material is carried from the fuel rods by the liquid moderator coolant, which flows from the pressure vessel and through a piping system connected to the pressure vessel and thence ultimately to a turbine-generator for generating electricity in a manner well understood in the art.
However, the fuel rods may occasionally leak. Leaking fuel rods are undesirable because such leaking fuel rods may release the radioactive fission products into the liquid moderator coolant flowing over the fuel rods, thereby radioactively contaminating any reactor system components in fluid communication with the coolant. Such reactor components may be, for example, structures housed within the pressure vessel, piping interconnecting the components of the reactor system, cooling tower apparatus or the turbine belonging to the previously mentioned turbine-generator. Increased levels of radioactive contamination in these reactor system components increases the time required for servicing the components because the components are typically decontaminated prior to servicing in order to reduce radiation exposure to service personnel. Moreover, leaking fuel rods increase the risk of discharging higher levels of radioactivity into the environment surrounding the nuclear reactor through an off-gas system associated with the nuclear reactor. Therefore, in order to avoid such undesirable radioactive contamination of reactor system components and the environment surrounding the reactor, it is important to detect any leaking fuel rods. If a leaking fuel rod is detected, it is replaced with a non-leaking fuel rod.
A prior art method of leak-detection includes latching a transport mechanism to the fuel assembly and removing the fuel assembly from the reactor core, such as during routine refueling operations of the reactor core. The fuel assembly is transported to a remote test chamber and placed therein to perform the leak-detection procedure. After being transported to the test chamber, the transport mechanism is unlatched from the fuel assembly. Next, while the fuel assembly resides in the test chamber, a vacuum is drawn in the test chamber to produce a pressure differential across the fuel assembly that facilitates migration of the fission product out any leaking or breached fuel rods. This also prevents the fission products from "hiding-out" in the breached fuel rod. The fission product material migrating out any leaking fuel rod is detected as it travels past a suitable detector located in the test chamber. If the fuel assembly contains no leaking fuel rods, the latching mechanism is again connected to the fuel assembly for reloading the fuel assembly into the reactor core or into a spent fuel pool depending on whether the nuclear fuel in the fuel assembly is capable of further fissions. On the other hand, if the fuel assembly contains a leaking fuel rod, the fuel rod is preferably replaced with a non-leaking fuel rod before being reloaded into the reactor core or transported to the spent fuel pool. Although this procedure has proven satisfactory for detecting leaking fuel rods, it is time consuming because each fuel assembly to be tested is latched, transported to the test chamber, unlatched, tested, relatched and then returned to the reactor core or transported to the spent fuel pool. Such a time consuming procedure undesirably lengthens the time required to refuel the reactor core. Therefore, a problem in the art is to provide a leak-detection apparatus and method that is less time consuming.
Moreover, applicants have discovered that at least some of the fission product material leaking from a breached fuel rod may undesirably adhere to the exterior surface of the fuel rod such that the fission product is not detectable by the detector. In addition, applicants have discovered that some of the fission product material may continue to "hide-out" in the fuel rod and may not sufficiently migrate out the breached fuel rod in order for it to be detected, even when subjected to the previously mentioned vacuum which is drawn in the test chamber. Therefore, another problem in the art is to detect leaking fuel rods even though the fission product material leaking from the fuel rod may adhere to the exterior surface of the fuel rod and even though the fission product material may tend to "hide-out" in the fuel rod.
Another method for detecting leaking nuclear fuel assemblies is disclosed in U.S. Pat. No. 4,416,847 titled "Method and Apparatus for Detecting Failure of Nuclear Fuel" issued Nov. 22, 1983 in the name of Shozo Saito, et al. This patent discloses a method for detecting failure of nuclear fuel which method comprises supplying air into a sipper cap mounted on a fuel assembly to form an air layer, isolating the fuel assembly to be detected from other assemblies, soaking the fuel assembly in a water bath, discharging the water from the system, and sampling that includes introducing a predetermined amount of cooling water in the fuel assembly and discharging the water-to a sample receptacle. However, this patent does not appear to disclose a solution to the problem of adequately removing fission product material adhering to the exterior surface of the fuel rods and does not appear to disclose a solution to the problem of detecting fission product material that may tend to "hide-out" in the fuel rods.
Although the above-recited prior art disclose leak-detection apparatus and methods, the above-recited prior art do not appear to disclose a leak-detection system and method for suitably detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom.
Therefore, what is needed are a leak-detection system and method for detecting a leaking container, which leaking container may be a leaking nuclear fuel rod having a radioactive fission product material leaking therefrom.